1. Field of the Invention
The present invention relates generally to a mobile communication system and, in particular, to a method and apparatus for transmitting and receiving channel status information in a distributed antenna mobile communication system.
2. Description of the Related Art
FIG. 1 is a schematic diagram illustrating a conventional cellular system including three cells each centered around an antenna, e.g., an evolved Node B (eNB) having transmit and receive antennas.
Referring to FIG. 1, three cells 100, 110, and 120 are included in the cellular system. The cell 100 is centered around the antenna 130 and serves User Equipments (UEs) 140 and 150 in its coverage area. The antenna 130 provides the UEs 140 and 150 located in the cell 100 with mobile communication services. Because the UE 140 is located farther from the antenna 130 than the UE 150, the UE 140 is served by the antenna 130 at lower data rate than the UE 150.
In FIG. 1, each of the cells 100, 110, and 120 is configured in the form of Central Antenna System (CAS), i.e., where the cell is centered around the antenna. In the CAS, although multiple antennas are allocated to each cell, all of the antennas are located at the center of the cell to serve the UEs in the service area.
In the CAS, as illustrated in FIG. 1, reference signals are transmitted for measuring downlink channel condition for each cell. For example, in a 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) system, a UE measures a channel status between the UE and an eNB using the Channel Status Information Reference Signal (CSI-RS) transmitted by the eNB.
FIG. 2 illustrates a conventional resource block including CSI-RSs transmitted by an eNB.
Referring to FIG. 2, the x axis is the time axis, and the y axis is the frequency axis. The minimum transmission unit in time domain is an OFDM symbol, and a subframe 224 includes two slots 222 and 223, each including NsymbolDL symbols. The minimum transmission unit in the frequency domain is a subcarrier, and the system frequency band is divided into NBW subcarriers. NBW is equal to the system transmission band. The basic unit of time-frequency resource is a Resource Element (RE), which is defined by an Orthogonal Frequency Division Multiplexing (OFDM) index symbol index and a subcarrier index. Resource Blocks (RBs) 220 and 221 are defined as NsymbolDL contiguous OFDM symbols in the time domain and NSCRB contiguous subcarriers in the frequency domain. That is, an RB includes NsymbolDL×NSCRB REs. The minimum transmission unit of normal data or control information is RB.
A downlink control channel is transmitted in the first three OFDM symbols at the beginning of the subframe 224. A Physical Downlink Share Channel (PDSCH) is transmitted on resources remaining after the allocation for the downlink control channel in the subframe 224. A Demodulation Reference Signal (DM-RS) is the reference signal that is referenced by a UE to demodulate the PDSCH.
The subframe 224 carries CSI-RSs 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, and 219 for CSI-RS antenna ports. Here, the CSI-RSs 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, and 219 are transmitted by a UE with two CSI-RS antenna ports, such that the UE can perform downlink measurement at a location. The CSI-RSs 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, and 219 are defined per logically defined CSI-RS antenna port for channel status measurements of respective CSI-RS antenna ports.
When the same CSI-RSs 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, and 219 are transmitted through multiple physical antennas, the UE cannot discriminate between the physical antennas but merely recognizes a single antenna port.
In mobile communication systems including a plurality of cells, it is possible to transmit CSI-RSs 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, and 219 with cell-specific location information. That is, cells are assigned the time-frequency resources different in position for the CSI-RS to avoid interference between CSI-RSs of different cells.
However, in a CAS, because the antennas of each eNB are concentrated at the center of the cell, it is difficult to provide a UE located far from the center of the cell with services at a high data rate.